Tire belts for general passenger cars and lightweight trucks may receive high centrifugal force during driving and thus be detached because they are composed of a steel cord and thus have a high specific gravity. Accordingly, to prevent such an accident, the steel cord may be replaced by a lightweight fiber cord or a tire structure, so-called, “capply” which can fix the belt to improve durability.
As driving speed increases, an increased strength is required for the fiber cord or capply. In an attempt to increase the strength of the fiber cord or capply, in general, a plurality of fiber cords or capplies are used or the arrangement density thereof is increased.
The nylon-66 cord conventionally used as the fiber cord or capply cord is generally used in the form of a thick cord or a laminate of two or more cords due to insufficient strength. This form increases weight and volume, and is thus disadvantageous in terms of heat dissipation, thus causing deterioration in tire durability.
Korean Patent No. 2003-13904 designed in order to solve this problem discloses a hybrid cord (complex fiber cord) with excellent strength and adhesive strength which includes a cord with high strength composed of a mono-filament and nylon-66 with excellent adhesive strength disposed around the cord to reduce the amount of used cord and thereby a tire weight. However, such a structure in which the center of the hybrid cord is composed of only one aramid mono-filament is inapplicable to tire capply cords that frequently bend and stretch due to drawback of poor fatigue resistance.
In addition, inventions associated with aramid hybrid cords used as reinforcement belts for conventional highly functional tires have focused on the ratio between initial modulus and breaking modulus. Representative aramid hybrid-related prior patents include Japanese Patent Publication No. 01-247204 by Sumitomo Rubber Industries Ltd. and U.S. Pat. No. 7,222,481 by Michelin. The Sumitomo Rubber Industries Ltd. patent defines the ratio between initial modulus and final modulus to 2 to 9 and the Michelin patent defines the ratio between initial modulus and final modulus to 10 or more.
The patents associated with the modulus ratio have an ambiguous range because they do unclearly define the initial modulus. Theoretically, initial modulus is defined as a slope of tangent at a strain of zero on a stress-strain curve. However, when tangent is drawn at zero on a real tire cord measurement graph, a very small slope is obtained. To prevent this phenomenon, a certain area should be set and a tangent (tan α) slope value should be obtained. However, the patents do not disclose such an area. In addition, actually, it is unclear whether or not the slope of the tangent at a strain of zero accurately represents an initial modulus under an initial load condition of 0.5 cN/tex or 0.05 g/d in a general tensile test.
Due to the structural characteristics in which the collection force of fiber bundles constituting the cord is insufficient at a load less than a predetermined level under conventional initial load conditions or at infinitesimal intervals, stress is not evenly distributed and the cord is structurally deformed. For this reason, it is not sufficient to define an initial modulus of a material under the initial load condition during a tensile test.
This phenomenon makes the modulus at an initial infinitesimal interval lower than the desired value. In reality, when a tensile test is conducted at an initial load of 0.05 g/d applied only with an aramid cord and the slope of the tangent at the origin, that is, modulus at an infinitesimal interval at the origin is obtained, the ratio of initial modulus to final modulus may be obtained as the undesired value, e.g., 1:10.
In addition, Japanese Patent Publication No. 01-247204 and U.S. Pat. No. 7,222,481 associated with the aramid hybrid cord disclose that nylon is disposed in the core whereas aramid is disposed in the covering when the twist number of aramid plies is different from that of polyamide 66 plies. However, In reality, although very flexible fibers are cabled at different twist numbers, the core is unclearly distinguished from the covering.